1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦193 nm as well as a projection exposure apparatus with such an illumination system.
2. Description of the Related Art
In order to be able to further reduce the structural widths of electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. Lithography with very deep UV radiation, so called VUV (Very deep UV) lithography or with soft x-ray radiation, so-called EUV (extreme UV) lithography, is conceivable at wavelengths smaller than 193 nm, for example.
An illumination system for a lithographic device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least 4 pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided, and the point plasma light source is imaged in an annular illuminated surface by means of a condenser, which has five aspherical mirrors arranged off-center.
From U.S. Pat. No. 5,581,605, an illumination system has been made known, in which a photon beam is split into a multiple number of secondary light sources by means of a plate with concave raster elements. In this way, a homogeneous or uniform illumination is achieved in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of a conventional reduction optics. A gridded mirror is precisely provided with equally curved elements in the illumination beam path. The contents of the above-mentioned patents are incorporated by reference.
EP-A-0 939 341 shows an illumination system and exposure apparatus for illuminating a surface over an illumination field having an arcuate shape with X-ray wave length light. The illumination system comprises first and second optical integrators each with a plurality of reflecting elements. The first and second optical integrators being opposingly arranged such that a plurality of light source images are formed at the plurality of reflecting elements of the second optical integrator. To form an arcuate shaped illumination field in the field plane according to EP-A-0 939 341 the reflecting elements of the first optical integrator have an arcuate shape similar to the arcuate illumination field. Such reflecting elements are complicate to manufacture.
EP-A-1 026 547 also shows an illumination system with two optical integrators. Similar to the system of EP-A-0 939 341 the reflecting elements of the first optical integrator have an arcuate shape for forming an arcuate shaped illumination field in the field plane.
In EP-A-0 955 641 a system with two optical integrators is shown. Each of said optical integrators comprises a plurality of raster-elements. The raster elements of the first optical integrator are of rectangular shape. The arc-shaped field in the field plane is formed by at least one grazing incidence field mirror. The content of the above mentioned patent-application is incorporated by reference. All above mentioned illumination systems have the disadvantage that the track-length of the illumination system is large.
It is therefore an object of the invention to overcome the disadvantages of the illumination systems according to the state of the art, to provide an illumination system for microlithography that fulfills the requirements for advanced lithography with wavelength less or equal to 193 nm and which is of compact size.